Consider going to a 14T, as it wears a lot better and better chain life.

The 38T would then be a 41T to keep ratio similar. All that is still too high, so consider going up to a ~45T+ for better accel and cruising. There's no perfect gear, but you can improve on what you got for your conditions.

I currently have a 14x45, and can still do 100mph per gps. My friend has 15x48 on his FS.

I used 15/41 or 14/41
Do'nt know what gear I use for 60 km/h...
I think 4th for 14t 15/41 4th gear wil be to low revs and 3 on the high side for cruising..
I just bought a new chain (+4 links=112) and wil use 41/43 rear sprockets 14/15 fronts
Mpg I get a good 140/145 km (90 miles+-) on 1 tank of fuel I do not ride superslow though

Stock is 14-38 on u.s. bikes .I went with 13-40 which equals +6 on the rear.This brings 3rd and 4th gears to life and makes 5th and 6th usable.Your problem might be mapping , it smooths the power out alot .2cents Skred

I like the mileage compared to my previous sumo bikes The fs 650 had a 1.5 liter bigger fuel tank but couln't go as far on 1 tank of fuel than the 570 before the fs650 I had a husqvarna smr450 that was realy shit fuel wise sometimes I ran out of fuel with less than 100km on the trip (reset 0 when fueled) not riding it very hard...
I pushed that bike 3/4 times